The present invention relates to a method for interconnecting layers in a semiconductor device, which enables a low-resistance contact.
According to the increased integration of semiconductor devices, the size of a contact hole is decreasing to below 1 .mu.m. As a result, a structure of the interconnecting layer having an ohmic contact of low resistance is greatly required.
FIGS. 1A-1E are cross-sectional diagrams illustrating a method for interconnecting layers in a semiconductor device according to the prior art.
Referring to FIG. 1A, after field oxide layers 12 are formed on a semiconductor substrate 11 by a conventional local oxidation of silicon (LOCOS) method, impurities are injected into semiconductor substrate 11 exposed to a surface between field oxide layers 12 to thereby form an impurity diffusion region 13. Referring to FIG. 1B, an insulating material such as SiO.sub.2 is vapor-deposited on the resultant structure using a general chemical vapor deposition (CVD) method to thereby form an insulating layer 14.
Referring to FIG. 1C, after a photoresist pattern (not shown) for forming the contact hole on insulating layer 14 is formed, the insulating layer is anisotropically etched using the photoresist pattern as an etching mask until the surface of impurity diffusion region 13 is exposed, thereby forming contact hole 15.
Referring to FIG. 1D, a titanium (Ti) layer 16 is formed by depositing titanium to a thickness of 300 .ANG.-900 .ANG. on the sides of contact hole 15, on impurity diffusion region 13 exposed by contact hole 15 and on insulating layer 14 using a sputtering method. Thereafter, a titanium nitride (TIN) layer 17 is formed by depositing titanium nitride to a thickness of 600 .ANG.-2000 .ANG. using the sputtering method. Subsequently, the above resultant structure is heat-treated in a nitrogen (N.sub.2) atmosphere, at a temperature above 450.degree. C.
Here, the TiN layer 17 is a barrier layer for preventing a reaction between a silicon layer constituting the impurity diffusion region 13 and an aluminum (Al) layer to be formed in a following process. Also, the Ti layer 16 is an ohmic contacting layer for enabling the low resistant contact between the TiN layer 17 and the impurity diffusion region 13.
Referring to FIG. 1E, an Al layer 18 filling the contact hole is formed by depositing aluminum on the above resultant structure using the sputtering method.
According to the method for interconnecting layers in the above-described conventional art, after the TiN layer is formed, the resultant structure is heat-treated at a temperature above 450.degree. C. As a result, first, the adhesion between the impurities diffusion region and the barrier metal layers (TiN/Ti layers) can be improved. Second, as shown in FIG. 1D, a low resistance contacting layer, such as a Ti.sub.x Si.sub.y layer 19, is formed on the interface between the barrier metal layer and the impurity diffusion region 13, thereby improving the interconnection property of the layers.
Shinichi Ogawa et al. report that when a heat-treatment process is performed after the contact hole is formed on a silicon (Si) substrate doped with impurities and a Ti layer is formed in the interior of the contact hole, Ti.sub.x Si.sub.y is formed on the Ti/Si interface. Higher heat-treating temperatures facilitate the formation of a crystal phase Ti.sub.x Si.sub.y. As a result, the barrier height of the Ti/Si interface is decreased (see: "Interface Microstructure of Titanium Thin-film/Silicon Single-crystal Substrate Correlated with Electrical Barrier Heights," Journal of Applied Physics, Vol. 70, No. 2, 15 Jul. 1991, p827-832).
Also, as the barrier height decreases, the contact resistance decreases. Therefore, in order to improve the layers' contacting resistance property, it is desirable that the heat treatment be performed at a high temperature of about 500.degree. C., at which the barrier height can be sufficiently decreased.
However, according to the method for interconnecting layers of the above-described conventional art, during the heat treatment in a reaction furnace for forming a low resistance contact layer between the TiN/Ti layer and the Si substrate, the TiN/Ti layer is changed into Ti.sub.x O.sub.y and TiO.sub.x N.sub.y layers by the oxygen existing in the reaction furnace. This phenomenon is severe when the heat treatment temperature is above 500.degree. C. Here, surface oxidation of the TiN layer is increased and much Ti.sub.x O.sub.y is remarkably formed on the interface between the Ti layer and the TiN layer. As a result, the layers' contacting resistance property deteriorates.
On the other hand, a step coverage of Ti/TiN layers formed by the sputtering method is poor. Especially, in the case of a contact hole which has a high aspect ratio and is small in diameter, very thin Ti/TiN layers are formed at the corners of the bottom of the contact hole. If the heat treatment is performed with such a thin Ti/TiN layer formed at the corners of the bottom of the contact hole, the Ti/TiN layer is completely oxidized. As a result, the contacting resistance at the corners of the contact hole is remarkably increased. Therefore, the deterioration of the contacting resistance property caused by the oxidation of the Ti/TiN layer generated during the heat treatment for forming the low-resistance contact layer is more severe for smaller contact holes.
As described above, in order to form a silicide which is stable in the electric property for improving the layers' interconnection property, the heat treatment must be performed at the high temperature and the oxidation of the barrier layer (TIN) and the ohmic contacting layer (Ti) during the heat treatment must be prevented.